Electroluminescent diodes (or light-emitting diodes, "LED"s) based on small emitting molecules vapor deposited as thin films from vacuum were reported by Tang et al (C. W. Tang and S. A. Van Slyke, Appl. Phys. Lett. 51, 913 (1987)); double-layer devices emitted 1000 cd/m.sup.2 with an external efficiency of 1%. Such organic small molecule LEDs have been extensively developed; Kido et al. (J. Kido, N. Nagai and Y. Okamoto, IEEE Transactions on Electron Devices 40, 1342 (1993)) reported luminance up to 40,000 cd/m.sup.2.
In organic LEDs, the active organic luminescent layer(s) are sandwiched between a transparent anode, such as indium/tin-oxide (ITO) as hole-injecting contact and a low work function metal (as electron injecting contact). This structure is described in detail by D. Braun and A. J. Heeger, Appl. Phys. Lett. 58, 1982 (1991); U.S. Pat. No. 5,408,109 and references therein).
Another light-emitting device is referred to as a light-emitting electrochemical cell ("LEC") and presents another approach to light-emitting devices fabricated from conjugated (semiconducting) polymers (Q. Pei, G. Yu, C. Zhang, Y. Yang, A. J. Heeger, Science 269, 1086 (1995), Q. Pei and F. Klavetter, U.S. patent application Ser. No. 08/268763). The light-emitting electrochemical cell comprises a blend of conjugated luminescent polymer and solid electrolyte, said blend serving as the active electroluminescent layer. The operating mechanism proposed for the polymer LEC involves the following steps:
(i) Electrochemical p- and n-doping in the regions adjacent to anode and cathode, respectively, upon application of voltage greater than E.sub.g /e where E.sub.g is the semiconductor energy gap (HOMO to LUMO) of the semiconducting polymer and e is the electronic charge; PA1 (ii) Formation, in situ, of a p-n junction within the active layer. PA1 (iii) Radiative recombination of p- and n-type carriers within the compensated p-n junction.
Because the ions must be mobile during the in-situ electrochemical doping in an LEC, an jonically conductive material needs to be present. The ionically conducting material can be the semiconducting and luminescent polymer, possibly enhanced by blending with a known ion transport polymer, for example, poly(ethylene oxide), PEO. Thus, conjugated luminescent polymers which contain ethylene oxide or other ion transport polymer repeat units in their structures are of particular interest for use in LECs.
Advantages of the polymer LEC include the use of stable metals as the electrodes (for example, gold) and the ability to fabricate the polymer LEC in a planar surface configuration (G. Yu, U.S. patent application Ser. No. 08/444998).
Both the polymer LED and the polymer LEC require semiconducting (conjugated) polymer in the active electroluminescent layer. Moreover, because competing non-radiative recombination of the injected electrons and holes will lead to reduced efficiency for electroluminescence, preferred semiconducting (conjugated) polymers must exhibit a high quantum efficiency for photoluminescence. Equally important, preferred semiconducting (conjugated) polymers must exhibit high stability, particularly stability against degradation because the shelf life and operating life of LEC and LED devices is ultimately dependent upon the stability of the active electroluminescent layer.
Several classes of luminescent polymers have been disclosed in the art heretofore. These include, for example, poly1,4-phenylene vinylene!, PPV (J. H. Burroughs, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. MacKay, R. H. Friend, P. L. Burns and A. B. Holmes, Nature 347, 539 (1990)) and soluble derivatives of PPV, such as MEH-PPV (U.S. Pat. No. 5,189,136) and BCHA-PPV (U.S. patent application Ser. No. 07/626463), PPPV (C. Zhang, H. von Seggern, K. Pakbaz. B. Kraabel, H.-W. Schmidt and A. J. Heeger, Synth. Met. 62, 35 (1994) and references therein) and the like, the soluble derivatives of polythiophene, for example the poly(3-alkylthiophenes) (D. Braun, G. Gustafssom, D. McBranch, J. Appl, Phys. 72, 564, (1992)), the soluble derivatives of polyphenylene (L. Hamaguchi and K. Yoshino, Jpn. J. Appl. Phys. L587 (1995)), and other semiconducting conjugated polymers which exhibit photoluminescence; blends of such semiconducting and luminescent conjugated polymers in hole-transporting or electron transporting polymers or molecules (C. Zhang H. von Seggern, K. Pakbaz, B. Kraabel, H.-W. Schmidt and A. J. Heeger, Synth. Met. 62, 35, (1994); C. Zhang, S, Hoger, K. Pakbaz, F. Wudl and A. J. Heeger, J. Electron. Mater. 23, 453 (1994)), and heterojunctions utilizing layers of semiconducting and luminescent conjugated polymers as donors and acceptors. Other materials proposed in the art include the polypara-phenylene!s, PPP, and the polythiophenes. The use of soluble derivatives of these basic structures enables relatively simple device fabrication by processing the active layer from solution (A. J. Heeger and D. Braun, U.S. Pat. No. 5,408,109 and references therein).
Alkyl and dialkyl derivatives of polyfluorene have also been used in polymer LEDs (M. Fukuda and K. Yoshino, Japanese Patent No. 02,269,734, Y. Ohmori, M. Uchida, K. Muro and K. Yoshino, Japanese Journal of Applied Physics 30, L1941(1991)).
Although a number of luminescent semiconducting polymers are known in the art, there is a specific need for stable materials with a sufficiently large electronic band gap that they emit blue light. Blue-emitting LEDs and LECs have been disclosed (G. Grem, G. Leditzky, B. Ullrich, and G. Leising, Advanced Materials 4, 36(1992), Y. Ohmori, M. Uchida, K. Muro and K. Yoshino, Japanese Journal of Applied Physics 30, L1941(1991), I. D. Parker, Q. Pei, and M. Morrocco, Appl. Phys. Lett. 65, 1272(1994)), but stable blue-emitting materials which yield LEDs and/or LECs with long lifetimes are not known.
Thus, there is a need for stable, large energy gap, semiconducting (conjugated) polymers which exhibit a high quantum efficiency for photoluminescence. In order to use such materials in light-emitting devices, it is necessary to fabricate optical quality thin films of the semiconducting, luminescent polymers. Thus, there is a need for stable, large energy gap semiconducting (conjugated) polymers which exhibit a high quantum efficiency for photoluminescence and which are processible from solution into optical quality thin films. Moreover there is particular need for conjugated luminescent polymers which are ionically conductive, for example, with ethylene-oxide repeat units present in their structures, because such materials are of particular interest for use in polymer LECs.
Stable, high quantum efficiency photoluminescent polymers are also important for applications which do not involve electroluminescence. Such materials have been demonstrated to be useful, for example, in dye lasers (D. Moses, U.S. patent application Ser. No. 07/904,731). More generally, luminescent materials with high photoluminescent efficiency are useful in a variety of display applications.